Have any of the "experts" you cited previously published the hypothesis about the use of the "X" in the literature, and if so, can you provide the citation.
Which is why no sources were mentioned in the article and it has been repeated here on SCF in this thread that there are no known citations; to me nor to anyone I contacted except for the article cited.
One point of the article was to put the "X" issue out there. All contacted were aware of the reason for the contact and that this would be floated in print. So as to speak "out loud" about what has been seen for years.
With mail thefts going on today, the "X" would be read as steal this item
Frankly I really don't care if the "x" means the sender of the registered item was a cute woman or hunky guy in the eyes of the clerk, but there is written about it in official sources. Yes that "x" was been around for decades and appears consistently during the time.
Here is another one with an "x" between a well known lawyer and a quite famous one. (I have the back but not handy on this '68 item. I just included the front in a talk I gave on 1341 usages Edit: to include $1 one Dollar Airlift for future search returns).
To spitball the rating of this zone 3, $9.95 in postage and fees item,
10 cents airmail
$4.25 first $10,000.00
$5.60 for next $35,000.00 at rate of 16 cents per thousand for zone 3 supplemental surcharges.
Even in 1968 dollars, $45,000.00 meant something.
USS Stamp Society journal, "The Specialist", March 2018 issue.
Thanks for the correction, I just checked my emails and that, February (2018), was the last anticipated date Leonard expected it to run. The title remained, Transition from Domestic Air Parcel Post to Priority Mail
Where can I find a copy of your article(s) on "x" markings? There ^^^ hoosierboy